1. Field of the Invention
This invention is directed to syringes for medical use and more specifically to syringes that can be used only once.
2. Description of Related Art
Reference is made to the following Letters Patent of the U.S.:
______________________________________ 4,687,467 (1987) Cygielski 4,699,614 (1987) Glazier 4,713,056 (1987) Butterfield 4,731,068 (1988) Hesse ______________________________________
A syringe has several basic parts, namely: a barrel, a plunger and a piston. The barrel normally houses the piston and supports the plunger in the form of a push-rod, or the like, for moving the piston axially within the barrel. The plunger exits the barrel at one end thereof. The barrel supports a hypodermic needle or the like at the opposite end. In use, a person withdraws the plunger thereby moving the piston away from the needle and transferring medicine into a cavity formed between the needle end of the barrel and the piston. When the correct amount of medicine has been withdrawn, the needle is injected into a patient and the plunger is moved toward the needle end, forcing the medicine from the cavity through the needle into the patient.
In recent years, considerable effort has been expended to provide a syringe for medical use that is both disposable and not reusable. Some of these syringes include a push-rod and piston that are detachably interconnected by some mechanical coupling or operator. The operator couples the push-rod and piston during an initial withdrawal to transfer medicine from a storage container into the syringe. When the push-rod moves toward the needle end during an injection, however, the operator decouples the push-rod and piston. Any attempt to withdraw the push-rod thereafter separates the push-rod and the piston, so additional medicine cannot be transferred into the syringe. The Hesse and Glazier patents are examples of this approach.
In other syringes a mechanism affixed to the barrel captures the piston or the push-rod when all the medicine has been injected into a patient. This prevents any subsequent withdrawal of the push-rod so that no additional medicine can be transferred into the syringe. The co-pending Plouff patent application and the Butterfield patent are examples of this general approach.
Syringes fill because a partial vacuum is produced within the syringe as the push-rod and piston are withdrawn during a filling operation. In the Cygielski patent, a cutter near the needle end of the syringe punctures a seal formed by the piston as a dose of medicine is administered to a patient. If the push-rod and piston are subsequently withdrawn, air leaks through the punctured seal so no partial vacuum forms and no medicine can transfer into the syringe.
Each of these proposed constructions have certain disadvantages. For example, the Plouff patent application and the Butterfield and Cygielski patents incorporate elements within the liquid cavity so these elements contact the fluid directly. This complicates the sterilization process. Including elements within the cavity complicates sealing construction and can lead to leakage. Syringes such as suggested by the Glazier and Cygielski patents contain mechanisms for achieving the one use objective. However, these mechanisms are somewhat complex and can increase manufacturing costs.